1. Field of the Invention
The present invention relates to a method and apparatus for measuring exhaust gas flow rate and it's application system for analyzing the exhaust gases from automotive engine or the like. Particularly, the invention relates to a method and apparatus being capable of correcting an error caused by pulsation of flow rate by using differential pressure type flow meter, for example, the Pitot tube flow meter or the like. Moreover, backward flow rate of the above mentioned gases also distinctly can be measured by the invention.
2. Background Art
A Pitot tube flowmeter is one of differential pressure type flowmeters for measuring flow rate of emission flowing in an exhaust pipe. In this Pitot tube flowmeter, gas flow rate converted to standard state Qpit(t) [m3/min] is given in the following formula (1). In the description of the invention below, (t) is added to the sign showing a time series value by time t, and, for example, gas flow rate is expressed as Qpit(t).
                              Qpit          ⁡                      (            t            )                          =                  K          ×                                    [                                                {                                                            Pg                      ⁡                                              (                        t                        )                                                              /                    1013                                    }                                ×                                  {                                      293.15                    /                                          Tg                      ⁡                                              (                        t                        )                                                                              }                                ×                                  {                                                                                    Pd                        ⁡                                                  (                          t                          )                                                                    /                      γ                                        ⁢                                                                                  ⁢                    g                                    }                                            ]                                                          (        1        )                where K: proportional coefficient            Pg(t): gas pressure [κ Pa]        Tg(t): gas temperature [K]        Pd(t): differential pressure        γg: gas density in standard state [g/m3]        
That is, when proportional coefficient K is determined preliminarily, from the measured values of temperature and pressure of emission flowing in the pipe and the differential pressure in the Pitot tube, the flow rate of the emission can be obtained.
The Pitot tube flowmeter is used in measurement of flow rate of emission from an engine, but the engine emission is known to generate pulsation in its exhaust pressure. The pulsation is an intrinsic problem of engine emission, and it has been attempted to elucidate radically the mechanism and effects of pulsation in various fields. When measuring continuously the flow rate of engine emission by using the Pitot tube flowmeter, since the emission pressure (differential pressure) is measured directly, if pulsation occurs, its effect is directly applied, and the measuring error may be increased.
FIG. 8 is a diagram showing the ratio of measured value of flow rate of gas having pressure fluctuations measured by using a Pitot tube and true value of gas flow rate. FIG. 9 is a diagram showing output waveform of flow rate measured by using the Pitot tube, in which an example of measurement at a constant flow rate is shown. Herein, a short cylinder engine different in displacement is connected to the gas passage, and pressure fluctuations are caused by rotating this short cylinder engine. As shown in FIGS. 8 and 9, when a pulsation of a certain frequency is applied to the gas flow, a large difference is caused between the measured value by the Pitot tube flowmeter and the true value of gas flow rate, and as the pulsation amplitude is larger, it is known that the difference is increased between the measured value by the Pitot tube flowmeter and the true value of gas flow rate.
Although pulsation at high frequency are removed by catalyst tube or muffler, when pulsation at low frequency are caused during idling of the engine or the like, pulsation frequency is lowered, and the Pitot tube flowmeter is strongly influenced by pulsation, and the error is increased.
As proposed, for example, in Japanese Laid-open Patent No. H10-318810, to decrease pulsation effects of engine, it has been attempted to install a buffer tank for absorbing pulsation of emission at the upstream side of the exhaust tube having the flowmeter, or to vary the capacity of the buffer tank depending on the rotating speed of the engine, thereby varying the frequency of the pulsation to be decreased. While the engine is idling, without using the output of the flow rate measured by the Pitot tube flowmeter, a certain preset fixed value may be used.
In any of the conventional measuring methods, the flow rate of emission exhausted from the engine could not be measured in-situ. That is, when using the fixed value instead of the measured value in idling state, since the fixed value completely ignoring the measured value of differential pressure is outputted in idling state, the flow rate of emission exhausted in idling state cannot be measured at all.
Further, as disclosed in Japanese Laid-open Patent No. H10-318810, when the buffer tank is installed at the upstream side of the exhaust tube, it is not only difficult to remove pulsation completely, but also the apparatus is complicated and increased in size because it is required to form a buffer tank of variable capacity and install a mechanism for varying the capacity of the buffer tank depending on the engine rotating speed. By using the buffer tank, since the pulsation is decreased by adjusting the flow rate of the emission to be measured, the flow rate (to be measured) having pulsation cannot be measured in-situ.
The invention is devised in the light of the above problems, and it is hence an object thereof to present a differential pressure type emission flow rate measuring method and emission flow rate measuring apparatus capable of correcting measuring error occurring due to pulsation appropriately and enhancing the measuring precision of flow rate when measuring the flow rate of emission having pulsation.
A Pitot tube flowmeter is known as one of differential pressure type flowmeters for continuously measuring flow rate of emission exhausted from an automotive engine and flowing in an exhaust pipe. In this Pitot tube flowmeter, emission flow rate converted to standard state Q(t) [m3/min] is given in the following formula (6).
                              Q          ⁡                      (            t            )                          =                  α          ×                                                                      P                  ⁡                                      (                    t                    )                                                  101.3                            ×                              293.15                                  T                  ⁡                                      (                    t                    )                                                              ×                                                Δ                  ⁢                                                                          ⁢                                      P                    ⁡                                          (                      t                      )                                                                      ρ                                                                        (        6        )                where α: proportional constant            ΔP(t): differential pressure of Pitot tube [kpa]        P(t): emission pressure [kpa]        T(t): emission temperature [K]        ρ: emission density in standard state [g/m3]        
That is, when proportional constant α is determined preliminarily, from the measured values of temperature and pressure of emission flowing in the pipe and the differential pressure in the Pitot tube, the flow rate of the emission can be obtained.
However, the Pitot tube flowmeter is known to have effects of pulsation of emission, and it is warned that the measuring error of emission flow rate increases due to so-called square root error occurring when calculating the emission flow rate from the differential pressure.
Accordingly, as disclosed in Japanese Laid-open Patent No. H10-318810, it has been attempted to use an apparatus for measuring the flow rate of emission by using a differential pressure sensor of flowmeter (Annubar flowmeter) while suppressing pulsation of emission by the buffer tank.
Further, as disclosed in Japanese Laid-open Patent No. 2000-46612, by processing the piping, a space filter is composed, and the pulsation frequency is determined, or as disclosed in Japanese Laid-open Patent No. 2001-208584, the pulsation frequency is determined by using an ultrasonic flowmeter.
However, in the emission flow rate measuring apparatus disclosed in Japanese Laid-open Patent No. H10-318810, the field of measurement is varied, by absorbing pulsation of emission by the buffer tank, it cannot be measured in-situ. Such technique is not suited to the present situation and future demand because elucidation of pulsation itself and transient measurement are demanded. It is also impossible to eliminate pulsation of emission completely by the buffer tank, and it further requires to form a buffer tank of variable capacity and mechanism for varying the capacity of buffer tank depending on the engine rotating speed, and the apparatus itself becomes complicated and larger in size. In the gas flow measuring apparatus disclosed in Japanese Laid-open Patent No. 2000-46612, or in the flow rate measuring apparatus disclosed in Japanese Laid-open Patent No. 2001-208584, the apparatus itself is large and complicated.
On the other hand, the Pitot tube flowmeter is designed to calculate the emission flow rate by making use of the proportional relation of the emission flow rate and square root of differential pressure as shown in formula (6), and hence the differential pressure and the emission flow rate are in the relation graphically shown in FIG. 16. In FIG. 16, the axis of abscissas denotes the differential pressure [kPa], and the axis of ordinates represents the flow rate [L/min]. According to the studies by the present inventors, the differential pressure changes in the time course along with pulsation of emission, and at the time of pulsation a in FIG. 16, for example, if the response frequency of the differential pressure sensor for detecting the differential pressure of the Pitot tube is low, measured values of differential pressure are averaged, and the emission flow rate (about 1300 L/min in FIG. 16) corresponding to the averaged measured value of differential pressure (e.g. about 1.4 kPa in FIG. 16) is obtained, and thus obtained emission flow rate is found to be higher than the accurate emission flow rate (about 1150 L/min in FIG. 16) obtained by averaging after converting the differential pressure into emission flow rate.
The invention is based on the above-mentioned findings, and it is hence other object thereof to present an emission flow rate measuring method and apparatus and an emission measuring system using the same apparatus, capable of measuring the flow rate of emission exhausted from an engine in real time and at high precision even in the event of pulsation, without changing the field of measurement, and also capable of reducing in size easily.